1. Field of the Invention
This invention relates to an internal combustion engine mounted on a personal watercraft for operation in water. More particularly, the invention relates to an improved balance shaft structure within the internal combustion engine.
2. Description of the Background Art
The personal watercraft, or small-sized planing boat, is constructed such that an internal combustion engine for driving a jet propulsion pump is mounted in a boat body enclosed by a hull and a deck. A driver and up to two other crew members rides on the deck, so that an in-boat space, constituted by the hull and the deck, is narrow. The internal combustion engine is stored in a substantially closed and sealed state within the narrow space between the hull and the deck. Therefore, a compact internal combustion engine is required.
The internal combustion engine must also produce minimal vibration. Thus, a secondary balance shaft for preventing a secondary vibration of a vibration caused by the reciprocation of pistons of the internal combustion engine is provided. A balance shaft of the secondary balance shaft is arranged in parallel with a crankshaft, and is rotated by means of interlocking gears therewith. However, since the balance shaft is arranged on the side of the crankshaft, the internal combustion engine is expanded in the width direction by the amount of space required for the balance shaft. Such an internal combustion engine is disclosed, for example, in JP-A No. 35201/2003.
A balance shaft in JP-A No. 35201/2003 is arranged in parallel with a crankshaft on a dividing plane of a crankcase and a cylinder block. The crankshaft is supported by a crankshaft bearing formed on a dividing plane of mutually opposed ribs of the crankcase and the cylinder block, whereby the crankshaft is permitted to rotate. The balance shaft is supported by a balance shaft bearing formed on the same pair of ribs, whereby the balance shaft can be rotated.
The balance shafts are supported at the respective ends thereof by both balance shaft bearings of the pair of ribs. The pair of ribs are constantly distant by an equal distance in parallel, and the distance between both balance shaft bearings of the pair of ribs is equal to distance between both crankshaft bearings.
Therefore, the portion of the engine which is enlarged by the presence of the balance shafts exists at least over distance between both crankshaft bearings. As a result, it is difficult to make the internal combustion engine more compact in size.
The invention is made in view of such a problem, and the object of the invention is to provide an internal combustion engine for a personal watercraft in which the internal combustion engine is small in size, the compact size achieved by limiting the enlargement of the portion of the engine that accommodates the balance shaft of the internal combustion engine.